Quasi-optics describes the application of optical techniques to microwave and millimeter wave frequencies. The term "quasi-optical propagation" is generally defined as propagation of electromagnetic energy in free space by a beam of radiation that is a relatively small number of wavelengths in the transverse dimensions. The conditions for such propagation, which becomes increasingly effective at millimeter wavelengths, can be established by either internally or externally exciting a resonator that consists of two reflecting mirrors. For a quasi-optical resonator of length L, resonance occurs at all frequencies which yield a wavelength, inside the resonator cavity, that satisfies ##EQU1## where m is any positive integer (axial number), .lambda. is the wavelength of the electromagnetic energy inside the resonator and L is the distance between the two reflecting mirrors. Thus, there are discrete spectral lines in the resonator, each having a fairly high quality factor (Q) and, consequently, low bandwidth.